Touch DNA Method

The touch DNA method—named for the fact that it analyzes skin cells left behind when assailants touch victims, weapons or something else at a crime scene—has been around since 2003. Touch DNA is left behind when people touch things, because they naturally shed skin cells as they touch things, and those cells contain the genetic material. The forensics team looks for DNA in places where the offender might likely have touched articles of clothing or the victim’s body. It’s not a stain that you can see.

To attempt to gather Touch DNA, scientists will scrape the surface of those areas where they think the offender may have touched with a sharp blade to see if they can find DNA. The technique has dramatically increased the number of items of evidence that can be used for DNA detection. Touch DNA doesn’t require you to see anything, or any blood or semen at all. It only requires seven or eight cells from the outermost layer of our skin.

Here’s how it works: Investigators recover cells from the scene, then use a process called polymerase chain reaction (PCR) to make lots of copies of the genes. Next, scientists mix in fluorescent compounds that attach themselves to 13 specific locations on the DNA and give a highly specific genetic portrait of that person. The whole process takes a few days.

These 13 locations were carefully chosen because they are highly variable between people and do not give away any specific information, such as race, gender, personal health or genetic disease. The reason: authorities don’t want personal health information being used for law-enforcement purposes, such as interrogations. The chance of DNA profiles from two different people having the same genetic signature is vanishingly small.

The trick to finding these cells: context. If clothing is removed from the victim, as it was in the Ramsey case, a forensic specialist could try to guess where it might have been handled—perhaps the waistband of a pair of pants—and swab those areas with a Q-tip or a blade. But in cases like the JonBenet Ramsey murder, which has tripped up authorities for over a decade, it can provide information that leads to a killer—or at least exonerates the innocent. (~snippets taken from noted source~)